Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Stone Consolidating Materials: A Status Report
Stone Consolidating Materials: A Status Report
Pages 287-311
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From page 287... ...
Evaluation of stone consolidants usually requires both laboratory and field tests to determine their initial and long-term performances. ASTM Standard E 632, Recommended Practice for Development of Accelerated Short-Term Tests for Prediction of the Service Life of Building Materials and Components, can be used to provide guidance on the test program.
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From page 288... ...
We deck is ~neIaDy bobeved to reset hom ~ss~ubon ~ We mutely cements We was gone together OI ~~ eta ~ We ~te~l~ bonds hom Accessed tensHe stresses caused by sum processes as sat ~st~lizadon ad ~~ fusion. ted bonds ~ ~ ~ porous stone con be mpres~ted schematically as ~ Fife 1.
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From page 289... ...
Treated stones are composite materials, and their properties reflect the properties of their individual ingredients, the interactions among them, and their spatial distributions. Since the properties and longterm performance of such composites cannot be satisfactorily predicted, the selection of consolidating materials and treatments must usually be based on laboratory and field tests of treated stones.
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~ 1 —I I requirements and criteria | ~ , _ 2 Characterize the component or material 3 9 4 Identify the expected type and range of degradation factors including those related to weathering biological, stress, 1ncompat1b111ty and use factors _ __ IPostulate ho, degradation characteristic of 1nuse performance-can be induced by accelerated aging tests I dent i fy pos s 1 bl e degra da t i on mec ban i smut 5 _ __, 6 PART 2 - PRE-TESTING PART 3 - TESTING Def i ne performance I requ1 remeets for I predictive service 1 life tests I 8 Design and perform prel1m1nary accelerated aging tests to demonstrate rapid failures caused by indiv1dually applied extreme degradat10n factors and to confirm degradation mechanisms . Design and perform predictive service life tests using the degradation factors of ~ importance to determine the dependence of the rate of degradation on exposure conditions PART 4 - INTERPRETATION AND REPORT I NG OF DATA 11 ~ ~ .
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From page 291... ...
Ike review is based on a previous paper by James R Clifton, which gives a more extensive bibliography.2 291 STONE CONS-OLIDANTS In this review, stone consolidants are divided into four mam groups: inorganic materials, alkoxysilanes, synthetic organic polymers, and waxes.
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From page 292... ...
Freeze-thaw cycles Wind Biological Factors Microorganisms Fungi Bacteria Stress Factors Stress, sustained Stress, periodic Stress, random Physical action of water, as rain, hail, sleet, and snow Physical action of wind Combination of physical action of water and wind Movement due to other factors, such as settlement or vehicles Incompatability Factors Chemical Physical Use Factors Design of system Installation and maintenance procedures Normal wear and tear Abuse by the user
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From page 293... ...
In practice, however, little concern seems to be given to chemical compatibility between the consolidants and the stone. Little success has been achieved in consolidating stone with inorganic materials, and in some cases their use has greatly accelerated decay.7-9 Some of the reasons given for the poor performance of inorganic consolidants are their tendencies to produce shallow, hard cruets, 7 i0 ii the formation of soluble salts as reaction by-products,7 i0 i2 i3 60 94 growth of precipitated crystals, 8 and the questionable ability of some of them to bind particles of stone together.~4 i5 Of these, the most difficult problem to overcome is the formation of shallow, hard surface layers by inorganic consolidants because of their poor penetration abilities.
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From page 295... ...
Further, soluble salts are formed when both limestone and calcareous sandstone are treated with silicofluorides.~° These soluble salts have caused damage through salt recrystallization processes. Penkala recently carried out a systematic study of several stone treatments and also found that fluorosilicates were not effective consolidants.25 Alkaline Earth Hydroxides Calcium Hydroxide Aqueous solutions of calcium hydroxide (its saturated solution is often called limewater)
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From page 296... ...
The newly produced calcium carbonate is susceptible to the same deterioration processes as the calcareous stone. For example, it can react with sulfur trioxide to form calcium sulfate, which is relatively soluble compared to calcium carbonate.
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Barium sulfate can be precipitated in a stone by an analogous method. An aqueous solution of a barium monoester of sulfuric acid hydrolyzes slowly when a base is added, releasing barium and sulfate ions.36 The precipitation of barium carbonate and barium sulfate from homogeneous solution is a promising approach.
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From page 298... ...
P Laurie received a patent in 1925 for producing such a material to be used for stone consolidation.5i Other early researchers on the use of alkoxysilanes to consolidate stone are Cogan and Setterstrom.52 53 Alkoxysilanes have been commonly used since 1960 in Germany.2i And recently, a promising alkoxysilane consolidating material called Brethane has been developed at the United Kingdom Building Research Establishment.45 A]
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From page 299... ...
It is claimed that their consolidating ability can be increased by using a mixture of alkoxysilanes.43 Some confusion appears in the literature regarding the differences between silicon esters, silicones, and alkoxysilanes Silicon esters are partially polymerized alkoxysilanes that still have ester groups attached to silicon. Silicones are polymerized alkoxysilanes that are dissolved in organic solvents and used as water repellents.43 Performance of Alkoxysilanes Weber and Price have observed that alkoxysilanes can usually penetrate porous stones to a depth of 20 to 25 mm.44 50 The newly developed Brethane has been reported to penetrate as deeply as 50 mm.45 No noticeable polymerization occurs with Brethane for at least three hours after it is mixed with a solvent and catalysts.42 50 Marschner reported that alkoxysilanes improved the resistance of sandstone to sodium sulfate crystallization.5498 However, she also observed that their performance varied from sandstone to sandstone and also depended on the compatibility between the solvent and the specific stone being treated.
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From page 300... ...
Viscous monomers are diluted with solvents so that deep penetration can be achieved.58 However, solvents that evaporate rapi~y (many common organic solvents) have been found to draw organic consolidants back to the surface of a stone, resulting in the formation of hard, impervious surface crusts.58 59 Munnikendam recommends the selection of organic consolidants whose solidification does not depend on evaporation of solvents.4i Among synthetic polymer systems, both thermoplastics and thermosets have been used to consolidate stone.
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From page 301... ...
Some organic consolidants have been found to improve the mechanical properties of deteriorated stone significantly. Many organic polymers are susceptible to degradation by oxygen and ultraviolet radiation, but this should only affect the materials on the surface of a treated stone.60 Riederer reported that the surfaces of some structures in Germany that had been consolidated with organic polymers in 1965 had exhibited deep channel erosion by 1975.2i Apparently water gradually eroded the consolidated surface and, once the protective surface layer was pierced, the untreated stone was eroded rapidly.
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From page 302... ...
However, as is the case with alkoxysilanes, stone impregnated with methyl methacrylate will probably weather differently from untreated stone. In addition, erosion through the treated stone could contribute to the development of an unsightly appearance.2i Acrylic Copolymers Copolymers are produced by joining two or more different monomers in a polymer chain.66 A commercially available acrylic copolymer used for stone consolidation is produced Tom ethyl methacrylate and methyl acrylate.55 67 Other acrylic copolymers that have been studied for stone conservation include copolymers of acrylics and fluorocarbons 68~69~99 and of acrylics and silicon esters.4~ 55 The acrylic copolymers are dissolved in organic solvents and then applied to stone.
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From page 303... ...
The resulting cross-linked polymers have excellent adhesion to stone and concrete and excellent chemical resistance. Lee and Neville, and Gauri, are recornrnended sources for information on the chemistry, curing, and applications of epoxies.73 75 Gauri developed a way to achieve deep penetration with viscous epoxy resins and at the same time to avoid the formation of a sharp interface between the consolidated and untreated stone.75 76 i00 Specimens were soaked in acetone, then in a dilute solution of epoxy resin in acetone, Then in increasingly concentrated solutions.
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From page 304... ...
when exposed to sunlight.73 Therefore, epoxy should be removed from the surface of a treated stone before it cures. Other Synthetic -Organic Polymers Other synthetic organic polymers studied as possible stone consolidants include polyester,67 84 polyurethane,55 and nylon.85 Polyester has been shown to decrease the porosity of stone substantially84 and, therefore, may form an impervious layer that prevents the passage of entrapped moisture or salts.67 Manaresi and Steen observed that polyurethanes were poor cementing agents.5686 Steen also foment that a polyurethane film gradually became brittle when exposed to sunlight.87 Similarly, DeWitte found that nylon can produce a brittle film on the surface of stone.85 Waxes Waxes have been applied to stone for more than 2,000 years.
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From page 305... ...
These considerations point to the inadequacy of the present state of stone consolidation and conservation technology. For example, stone consolidants should be selected on the basis of an understanding of the deterioration processes of stone and treated stone, of the factors affecting the performances of consolidants, and of the compatibility of consolidants with specific stones.
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From page 306... ...
Consolidants might be used on structures of little historical or intrinsic value and in other cases where the benefits outweigh the risks involved.~02 For example, consolidants could be applied to deteriorated stone to delay the need to replace it with new stone. Any permanent consolidation effort involving important historic stone structures, however, should be carefully planned once carried out to minimize the risks.
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From page 307... ...
3. ASTM Standard E 632, Recommended Practice for Development of Short-Term Accelerated Tests for Prediction of Service Life of Building Materials and Components {American Society for Testing and Materials, Philadelphia, 1980~.
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From page 308... ...
E.V. Sayre, Direct Deposition of Barium Sulfate from Homogeneous Solution Within Porous Stone, pp.
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C.A. Price, Research on Natural Stone at the Building Research Establishment, Natural Stone Directory ~1977J.
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G Marinelli, Use of an Epoxy Aliphatic Resin in the Consolidation of Porous Building Materials Having Poor Mechnical Properties, pp.
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193-200, Intemational Symposium on the Deterioration of Building Materials {Athens, Greece, 1976J.
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